Formatting a lustre filesystem in RHEL6 barriers are enabled by default with our Lustre 2.0 (same for Lustre 2.1). The impact on small IO performance is very important if we don't specify nobarrier as an MDT mount option. A single fileop bench running with '-f 15' will take more than 2m30sec while the one with nobarrier as mount option on MDT will take less than a minute. Compiling a kernel in Lustre is also a good way to see the problem due to jbd2 barriers.
This problem had already been addressed in SLES with ldiskfs patch ext4-disable-write-bar-by-default.patch. I adapted a similar patch to RHEL6.1. I'll try to submit it as soon as possible.